1. Field of the Invention
This invention relates to the purification and controlled release of water by pervaporation, and specifically relates to the use of a hydrophilic membrane in an apparatus that allows the direct use of pure or impure water for irrigation, rehydration or drinking.
2. Description of the Related Art
There are known processes for purifying water, and the type of process used depends on the nature and amount of the impurities in the water. For example, impurities in particulate form and in solution may both need to be removed from the water. The aim is to purify the water so that it contains sufficiently low amounts of suspended particles, suspended microbes and dissolved salts to meet the quality requirements of water for drinking, for the production of food and drink, for agricultural irrigation and for industrial use.
Processes for the purification of water are usually classified as filtration, distillation or reverse osmosis. In conventional particle filtration processes, impurities in particulate form, such as suspended inorganic particles, are removed using porous constructions such as woven or non-woven fabrics. In cases where very small particles must be filtered, polymer membranes are used which are microporous, that is, the membranes have very small holes through which the particulates to be filtered cannot pass.
Aqueous solutions containing dissolved salts are usually purified by reverse osmosis or distillation. When the aqueous solution is in the form of seawater or brackish water, these processes are generally known as desalination. The process of reverse osmosis relies on applying pressure to solutions of ions across a semi-permeable membrane. If the applied pressure is greater than the osmotic pressure of the solution, purified water is collected from the side of the membrane not in contact with the solution. Reverse osmosis membranes let water pass through them but reject the passage of salt ions. In reality a small percentage, say 1%, of sea salts pass through the membranes. U.S. Pat. No. 5,547,586 discloses a method to desalinate seawater and brackish water utilizing an enzyme-assisted membrane. As opposed to reverse osmosis, distillation methods using seawater or brackish water can yield water with a very low amount of suspended particles and dissolved solids. However, the high latent heat of vaporization of water means that distillation processes require a high input of energy and therefore generally operate at a higher cost compared to reverse osmosis processes.
It has been known from U.S. Pat. No. 4,725,481 that a copolyetherester elastomer can be used, on its own or as a part of a bicomponent film of a hydrophobic layer and a hydrophilic layer of copolyetherester elastomers bonded together, to permit the differential transfer of water vapor to prevent buildup of moisture such as in a surgical drape or in waterproof apparel equipment.